Reservoirs containing heavy oil (e.g., oil having a viscosity above 1500 cP at reservoir temperature) sometimes have compositional gradients. Where such reservoirs are thick (e.g., having a vertical extent exceeding 25 meters), the effect of the compositional gradients may be amplified. For example, the compositional gradients may cause changes in viscosity as a function of depth, perhaps changes having several orders of magnitude. Therefore, modeling reservoir response to thermal stimulation, solvent flood, and/or other oil recovery mechanisms can be difficult without sufficient knowledge of the compositional gradients, including any breaks in the trend of compositional gradients that may indicate compartmentalization within the reservoir.